High Trigger Current Silicon Controlled Rectifier
An ESD protection circuit including an SCR having at least a PNP transistor and at least a NPN transistor such that said PNP transistor is coupled to an anode and the NPN transistor is coupled to a cathode. The circuit also includes a first resistor coupled between the anode and the base of the pnp transistor and a second resistor coupled between the cathode and the base of the npn transistor. A parasitic distributed bipolar transistor is formed between said first and second transistor to control triggering of the SCR.
This patent application claims the benefit of U.S. Provisional Application Ser. No. 61/079,221 filed Jul. 9, 2008, the contents of which are incorporated by reference herein.
FIELD OF THE INVENTIONThe present invention generally relates to circuits that provide improved electrostatic discharge (ESD) protection, and more particularly to method and apparatus for providing a high trigger current silicon controlled rectifier.
BACKGROUND OF THE INVENTIONSilicon controlled rectifiers (SCRs) have long been used over a broad range of technologies because of their superior performance. During an ESD event, the SCR is considered a superior device because the SCR acts as an almost ideal shunt element.
One concern in the industry about using SCRs as ESD protection devices is unintentional latch-up during normal operating conditions. Latch-up is an uncontrolled triggering of an (parasitic) SCR structure on the IC during normal operation, such that the supply voltage is shorted to ground. The holding currents of such (parasitic) SCR structures are specified in the industry as the minimum latch-up current. Typical values are a minimum of 100 milliamps, or up to 300-500 milliamps under severe operating conditions. A latch-up condition could lead to very high currents from the power supply that may permanently damage the IC.
One method to avoid latch-up in the SCR ESD protection devices is to provide serial coupled diodes between, for example, a pad and the anode of the SCR, such that the holding voltage is kept above the supply voltage. In other words, when the holding voltage is above the supply voltage (including some safety margin), the risk of a latch-up condition is avoided. Generally, there is a tendency in the industry to use lower voltages to power the IC's, yet there are circuit applications where even much higher voltages are required (e.g., automotive applications or IC's for certain functions in cellular phones). Accordingly, the higher the supply voltage, the more series diodes are required.
The use of the series diodes with the SCR has several disadvantages. A first disadvantage for such a high holding voltage is that a considerable number of serial coupled diodes would be a needed, which requires additional area on the IC. A second disadvantage is that the serial diodes add extra resistance during ESD stress, except for increasing the holding voltage. A third disadvantage is that a large number of series diodes may result in high leakage currents, due to a parasitic Darlington transistor to the substrate that amplifies an initial leakage current and becomes more problematic at higher operating temperatures.
In particular, each serial diode forms a stage of the Darlington transistor, and the stages are connected such that the leakage current of one stage is being amplified by the next stage, and so forth. This is called the Darlington amplifier in standard circuit theory, and the more of these Darlington stages are coupled, the more leakage current is generated. Moreover, during high ambient or operating temperatures of the chip, the leakage current increases, because there is more thermal carrier generation. As such, the series diodes pose a strong limit to the application of the SCR devices for also satisfying the above-mentioned latch-up concern.
Another solution is to increase the trigger current above the maximum current injected during the latch up test. This is known as high holding current silicon controlled rectifier (HHISCR) ESD protection device which is disclosed in prior art U.S. Pat. No. 6,803,633.
Therefore, there is a need in the art for an improved HHISCR ESD protection device having a structure occupying less area with increase in its triggering speed while providing for a high immunity to latch-up condition during normal operation.
SUMMARY OF THE INVENTIONThe present invention provides an electrostatic discharge (ESD) protection circuit having an SCR including at least a PNP transistor and at least a NPN transistor such that the PNP transistor is coupled to an anode and the NPN transistor is coupled to a cathode. A first resistor coupled between the anode and the base of the pnp transistor, a second resistor coupled between the cathode and the base of the npn transistor. A parasitic bipolar transistor is formed between the first and second resistor which functions to control triggering of the SCR during ESD.
The present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawings, of which:
It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention.
DETAILED DESCRIPTION OF THE INVENTIONReferring to
The resistors 205 and 206 have low resistance values preferably in the range of 2 and 100 Ohms to ensure the high trigger current. Additionally, the resistors 205 and 206 in this embodiment are active resistors. In other words, the resistors 205 and 206 are formed by doped regions in a well. Typical implementations of the resistors 205 and 206 are highly or lowly N or P doped regions in an Nwell or PWell substrates respectively. Also salicide/silicide block layers, special implant layers and like may preferably be used to adapt the resistance value. The resistors 205 and 206 shown here each have a first terminal and a second terminal such that the first terminal of resistor 205 is adjacent to the first terminal of the resistor 206 and the second terminal of the resistor 205 is adjacent to the second terminal of the resistor 206.
Referring to
Referring to
In the prior art, there is only current flowing either into G1 or G2. So, for example if the current flows only into G2, it will only turn on the pnp 203 first, then current of the pnp 203 will turn on the npn 204. In the distributed approach of the embodiment illustrated in
Thus, by combining the two resistors 205 and 206 as discussed above, the bipolar transistor 207 is formed which is used to improve the trigger of the SCR. One of the advantages of the present invention is that these resistors are used as a triggering element eliminating the need of an additional trigger element required in the prior art. Also, in the prior art, only one of the trigger taps, G1 or G2 of the SCR trigger during normal operation. Whereas, in the present invention with the distributed triggering as described above, both the G1 and G2 trigger taps trigger during normal operation, thus increasing the triggering speed of the SCR.
Referring to
Referring to
As described above with respect to
Further in the embodiment of
In order to trigger the SCR, there must be 0.7V over the resistors 206 and 205 to turn on the base emitter of the PNP 203 and NPN 205 of the SCR. So, as discussed above, in this embodiment, bipolar transistor 507a will not contribute any current to trigger the SCR, bipolar transistor 507b will only contribute half of the current flowing through the bipolar for turning on the SCR and the bipolar transistor 507c will contribute all the current flowing through the bipolar for turning on the SCR. In the previous embodiment as described in
Even though the above embodiments are illustrated and described with respect to an NPN bipolar transistor, one in the ordinary skill in the art would appreciate that the bipolar transistor can also be a PNP transistor and function similarly as discussed above with respect to the NPN bipolar transistor.
Referring to
Referring to
Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit and the scope of the invention.
Claims
1. An electrostatic discharge (ESD) protection circuit comprising:
- an SCR comprising at least a PNP transistor and at least a NPN transistor such that said PNP transistor is coupled to an anode and the NPN transistor is coupled to a cathode;
- a first resistor coupled between the anode and the base of the pnp transistor;
- a second resistor coupled between the cathode and the base of the npn transistor; and
- a parasitic bipolar transistor formed between said first and second resistor; said bipolar transistor functions to control triggering of the SCR during ESD.
2. The ESD circuit of claim 1 wherein said first resistor and said second resistor comprises highly doped regions; said second resistor is parallel to said first resistor.
3. The ESD circuit of claim 1 wherein said first resistor comprises a first highly doped N region in a P-well substrate and second resistor comprises a second highly doped N region in a P-well substrate.
4. The ESD circuit of claim 1 wherein said first resistor comprises a first highly doped P region in a N-well and second resistor comprises a second highly doped P region in a N-well
5. The ESD circuit of claim 1 wherein said first resistor comprises a first lowly doped N region in a first N-well and said second resistor comprises a second highly doped P region in a second N-well.
6. The ESD circuit of claim 1 wherein an isolation is formed between said first resistor and said second resistor.
7. The ESD circuit of claim 1 wherein a gate is formed between said first resistor and said second resistor.
8. The ESD circuit of claim 7 wherein a control circuit is coupled to the gate.
9. The ESD circuit of claim 8 wherein the control circuit comprise at least one of a diode, MOS, resistor, capacitor, inverter, SCR
10. The ESD circuit of claim 1 wherein said parasitic bipolar transistor comprising at least one distributed bipolar transistor.
11. The ESD circuit of claim 2 wherein the first resistor has a first and second terminal and the second resistor has a first and second terminal.
12. The ESD circuit of claim 11 wherein said first terminal of the first resistor is placed adjacent to the first terminal of the second resistor and said second terminal of the first resistor is placed adjacent to said second terminal of the second resistor.
13. The ESD circuit of claim 11 wherein the first terminal of the first resistor is coupled to anode, the second terminal of the first resistor is coupled to the base of the PNP, the first terminal of the second resistor is coupled to base of the NPN and the second terminal of the second resistor is coupled to cathode.
14. The ESD circuit of claim 11 wherein the first terminal of the first resistor is placed adjacent to the second terminal of the second resistor and the second terminal of the first resistor is placed adjacent to the first terminal of the second resistor.
15. The ESD circuit of claim 11 wherein the first terminal of the first resistor is coupled to anode, the second terminal of the first resistor is coupled to the base of the PNP, the first terminal of the second resistor is coupled to base of the NPN, the second terminal of the second resistor is coupled to cathode
16. The ESD circuit of claim 1 wherein the first resistor comprises at least one segment and said second resistor comprises at least one segment
17. The ESD circuit of claim 16 wherein the segments of the first resistor are coupled in parallel and the segments of the second resistor are coupled in parallel.
18. The ESD circuit of claim 16 wherein the segments of the first resistor are interleaved with the segments of the second resistor.
19. The ESD circuit of claim 1 wherein the first and second resistors are placed so the current flowing through the first resistor is parallel to the current flowing through the second resistor
20. The ESD circuit of claim 1 wherein the first and second resistors are placed so the current flowing through the first resistor is anti-parallel to the current flowing through the second resistor
Type: Application
Filed: Jul 9, 2009
Publication Date: Jan 14, 2010
Patent Grant number: 7986502
Inventor: Bart Sorgeloos (Pittem)
Application Number: 12/500,185
International Classification: H02H 9/04 (20060101);